In order to pump oil from a well in which casing has been set, the casing is perforated by a suitable gun at those levels in which oil has been indicated to exist. After the casing has been perforated, a pumping tube is lowered on the end of the conventional drill pipe string to this particular level and within the pumping tube itself there is then lowered a pumping apparatus or tool. The lower interior of the pumping tube terminates in an annular seat adapted to receive in sealing relationship the lower end of the pumping apparatus. The pumping apparatus itself is operated by sucker rods which pass up through the pumping tube and pipe string and are reciprocated to operate a plunger within the pumping apparatus. Oil may thus be pumped through the perforations in the casing and up past the lower annular seat of the pumping tube through the pumping apparatus and thence upwardly through the pipe string to the surface of the well.
After pumping has been completed or if it is desired to change the pump or repair it, it is necessary to unseat the lower end of the pumping apparatus or tool from the annular seat defined by the pumping tube. Since, however, the pumping apparatus as well as the tube itself and the pipe string is filled with oil throughout the length of the pipe string, whereas the surrounding fluid in the well annulus defined between the exterior of the pumping tube and the interior of the casing is at a relatively low level, there exists a very large pressure differential rendering it extremely difficult to unseat the pump from the annular seat of the pumping tube. In fact, the sucker rods themselves through which the pulling forces apply to the pumping apparatus may very well break before sufficient force is applied to unseat the pump.
The foregoing condition may be overcome if the fluid within the pipe string and surrounding the pumping apparatus within the pumping tube could be passed in a reverse direction through the pump tool and out the lower end thereof past the seat the the annulus exterior of the pumping tube. If the fluid could be passed in this manner, the large hydrostatic head created within the pumping apparatus and pipe string could be "dumped" into the surrounding annulus of the well and thus equalize the pressures involved so that removal of the pumping apparatus from the annular seat could be very readily achieved.
It is important in a device of the foregoing type that the hydraulic unloading of the fluid within the pipe string to the exterior annulus would only operate in the event a force greater than that which might damage the sucker rods would be required to unseat the pumping apparatus; otherwise, inadvertent equalization of the hydrostatic pressures might occur during normal pumping.
In other instances of pumping operations, the pump tool itself may become stuck in the bore hole and in such event, it would be desirable to remove at least a portion the tool so that suitable fishing equipment could be brought in to remove the stuck portion.
In my U.S. Pat. No. 3,168,873 issued Feb. 9, 1965, there is disclosed a hydraulic unloading and circulating device which solves the foregoing problems. Essentially, this device provides a housing cooperating with a mandrel partially telescoped within the housing. The mandrel itself includes a straight through fluid passage having a lateral port which, when the mandrel is in a first telescoped position, is eclipsed by the housing so that no fluid can pass through the lateral port. However, when the mandrel is telescoped outwardly to a second position, a lateral port is exposed so that there is provided a passage for fluid from the exterior of the device to the interior straight through passage.
The lower end of the housing terminates in threads adapted to be threaded to the lower threads of a threaded joint in the oil well pump tool itself below the normal ball valve head. The upper end of the mandrel, in turn, is coupled to the upper threads of the threaded joint of the tool.
Shear pins are provided to hold the mandrel in its first telescoped position in the housing so that the lateral port is normally eclipsed and normal pumping operations can ent force is exerted on the rods to shear the shear pins and thus permit the mandrel to telescope upwardly to its second telescoped position thereby placing the respective fluids in communication with each other so that the pressures are equalized. The pump can then readily be removed from the lower annular seat of the pumping tube since it is no longer held thereon by the hydrostatic pressure of fluid in the pipe string.
While the foregoing tool as described has been satisfactory, there is still room for improvement in such a device. More particularly, it would be desirable if the device could be designed so as to avoid the necessity of having to replace shear pins each time the device is to be used.